Since its introduction in 1991, modern Li-ion battery technology has found a wide range of applications from portable electronics to transportation systems. Due to the increasing demand on advanced energy storage devices, high energy density technologies such as Li—S and Li—O2 are now being extensively researched. Li—S cells offer a 6-fold increase in specific energy over conventional Li-ion systems. Their implementation is currently limited by the dissolution and migration of polysulfides in conventional liquid electrolytes. The polysulfide shuttle phenomenon leads to a decrease in columbic efficiency and rapid electrode degradation. In addition, cycling a metallic lithium anode in liquid electrolyte raises safety concerns due to the dendrite-caused internal cell shorting and high flammability of organic solvents.
Solid state Li-ion conductors offer a solution to these issues by providing a typically impermeable membrane that prevents the penetration of lithium dendrites and the migration of polysulfides. Additionally, they offer improved electrochemical, mechanical and thermal stability. Ionic conductivity of solid electrolytes can be as high as liquid electrolytes with concentrated salts. For example, the sulfide based Li10GeP2S12 has an unprecedented ionic conductivity of 1.2×10−2 S cm−1 which is comparable to that of 1M LiPF6 in the carbonate solvents.
Solid electrolytes are single ion conductors; the lithium ion transference number is 1. With comparable ionic conductivities, the effective lithium-ion conductivity of solid electrolytes is much higher than that of liquid electrolytes. However a majority of the solid state conductors suffer from poor ionic conductivity (10−6 to 10−8 S cm−1), while the better ionic conductors are not stable with metallic Li anodes.
The practical use of solid electrolytes in batteries extends beyond the investigation of ionic conductivity. Chemical compatibility of solid electrolytes with electrode materials, interfacial resistance, and processablity of the solid electrolytes restrict the deployment of solid electrolytes in batteries. Thus, it is very challenging to have a single electrolyte that meets all above requirements for practical use in batteries.